1. Field of the Invention
This invention relates to a method of controlling the film thickness of vacuum-evaporated film and a device therefor.
2. Description of the Prior Art
The monochromatic film thickness control method and the two-wavelength film thickness control method are popular as the conventional optical film thickness control methods. The monochromatic film thickness control method is a method wherein monochromatic light is used as the measuring light and the variation in reflection factor corresponding to the film thickness from a monitor during evaporation is used to control the film thickness. The two-wavelength film thickness control method is a method wherein two color lights are used as the measuring light and the variation in the difference in reflection factor between the two wavelengths corresponding to the film thickness from a monitor during evaporation is used to control the film thickness.
In the monochromatic film thickness control method, film thickness is usually controlled at the peak or valley of the output variation in reflection factor corresponding to an integer times 1/4 of the wavelength of the monochromatic light. However, the vicinity of this extremal value is insensible in the variation in reflection factor for the variation in film thickness, and the controlled film thickness often contains an error from a predetermined film thickness. Thus, this method suffers from the disadvantage that it is poor in reproducibility.
On the other hand, the conventionally practised two-wavelength film thickness control method is a method of controlling film thickness by observing in two different wavelengths monitors to which the same film thickness adheres. That is, where a film thickness an integer times 1/4 of the design wavelength .lambda..sub.0 is to be controlled, if two wavelengths .lambda..sub.1 and .lambda..sub.2, one shorter than .lambda..sub.0 and one longer than .lambda..sub.0 and which satisfy the relation that 2/.lambda..sub.0 =1/.lambda..sub.1 +1/.lambda..sub.2, (and a wavelength longer than .lambda..sub.0 which satisfies such relation) are selected, the reflection factors at the wavelengths .lambda..sub.1 and .lambda..sub.2 become equal to each other when the film thickness is an integer times .lambda..sub.0 /4 and by utilizing this, the zero point of the output of the difference in reflection factor between the wavelengths is observed to control the film thickness of the evaporated film. The vicinity of this zero point is sensitive in the output variation of the difference in reflection factor for the variation in film thickness and accordingly, it is good in reproducibility. However, this zero point corresponds to a film thickness of 1/4 of the controlling wavelength .lambda..sub.0 where the controlled film includes no dispersion of refractive index, and the film thickness controlled at the zero point becomes greater than the target value where the controlled film is a high refractive index film having a high dispersion. That is, where alternate multi-layer films of .lambda..sub.0 /4 integer film construction comprising a high refractive index film and a low refractive index film are to be formed, there is the disadvantage that the film thickness ratio between the high refractive index film and low refractive index film does not become an integer.